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<h2><b>oacres()</b></h2>
<p>
<code>oacres (<i>gexpr, sexpr &lt,radii&gt</i>)</code>

<p> 
A Cressman objective analysis is performed on the station data to arrive
at a gridded result that represents the station data. 

<p> 
<ul>
<code><i>gexpr</i>&nbsp;&nbsp;&nbsp;</code>a valid grid expression<br>
<code><i>sexpr</i>&nbsp;&nbsp;&nbsp;</code>a valid station data expression<br>
<code><i>radii</i>&nbsp;&nbsp;&nbsp;</code>optional radii of influence. Defaults are: 10,7,4,2,1<br>
</ul>
<p>
The Cressman Analysis scheme is described in a 1959 paper in Monthly
Weather Review. Multiple passes are made through the grid with
increasingly smaller radii of influence. At each pass, a new value is
calculated for each grid point based on a correction factor that is
determined by looking at each station within the radius of influence.
<p>
For each such station, an error is defined as the difference
between the station value and a value arrived by interpolation from
the grid to that station. The correction factor is based on a distance
weighted formula applied to all such errors within the radius of
influence. The correction factors are applied to each grid point
before the next pass is made. 
<p>
Any grid boxes that do not have stations within the third specified
radius are set to the missing data value.

<p>
<h3>Usage Notes </h3><p>

<ol>
<li>The <code>oacres</code> function can be quite slow to execute,
depending on grid and station data density. 
<p>
<li>The actual values of the gridded expression are ignored, but 
the grid itself is used as a template to perform the analysis. The scaling of
the grid must be linear in lat-lon.
<p>
<li>The Cressman Analysis scheme can be unstable if the grid density is
substantially higher than the station data density (ie, far more grid
points than station data points). In such cases, the analysis can produce
extrema in the grid values that are not realistic. It is thus suggested
that you examine the results of <code>oacres</code> and compare them to
the station data to insure they meet your needs. 
<p>
<li>Objective analysis is a complex topic, and many schemes for doing
it have been developed over the years. The <code>oacres</code>
function is more of a quick-look function rather than a rigorous
analysis scheme. If you have specific analysis requirements, consider
doing your objective analysis outside of GrADS with a <a
href="udf.html">user-defined function</a>.
<p>
<li>See the related function <a href="gradfuncoabin.html"><code>oabin</code></a>.
</ol>

<h3>Examples </h3>
<p>
<ol>
<li>The simplest case: 
<p>
<dd><code>oacres(ts,ts.2)</code>
<p>
<li>To specify your own radii of influence: 
<p>
<dd><code>oacres(ts,ts.2,12,8,5,4,3,2,1)</code>
<p>
</ol>


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